If either
tv
or
tz
is NULL, the corresponding structure is not set or returned.

The use of the
timezone
structure is obsolete; the
tz
argument should normally be specified as NULL.
The
tz_dsttime
field has never been used under Linux; it has not
been and will not be supported by libc or glibc.
Each and every occurrence of this field in the kernel source
(other than the declaration) is a bug.
Thus, the following
is purely of historic interest.

The field
tz_dsttime
contains a symbolic constant (values are given below)
that indicates in which part of the year Daylight Saving Time
is in force.
(Note: its value is constant throughout the year:
it does not indicate that DST is in force, it just selects an
algorithm.)
The daylight saving time algorithms defined are as follows :

Of course it turned out that the period in which
Daylight Saving Time is in force cannot be given
by a simple algorithm, one per country; indeed,
this period is determined by unpredictable political
decisions.
So this method of representing timezones
has been abandoned.
Under Linux, in a call to
settimeofday()
the
tz_dsttime
field should be zero.

Under Linux there are some peculiar "warp clock" semantics associated
with the
settimeofday()
system call if on the very first call (after booting)
that has a non-NULL
tz
argument, the
tv
argument is NULL and the
tz_minuteswest
field is nonzero.
In such a case it is assumed that the CMOS clock
is on local time, and that it has to be incremented by this amount
to get UTC system time.
No doubt it is a bad idea to use this feature.

Macros for operating on
timeval
structures are described in
timeradd(3).

RETURN VALUE

gettimeofday()
and
settimeofday()
return 0 for success, or -1 for failure (in which case
errno
is set appropriately).

ERRORS

EFAULT

One of
tv
or
tz
pointed outside the accessible address space.

EINVAL

Timezone (or something else) is invalid.

EPERM

The calling process has insufficient privilege to call
settimeofday();
under Linux the
CAP_SYS_TIME
capability is required.

CONFORMING TO

SVr4, 4.3BSD.
POSIX.1-2001 describes
gettimeofday()
but not
settimeofday().
POSIX.1-2008 marks
gettimeofday()
as obsolete, recomending the use of
clock_gettime(2)
instead.